The invention relates to a method for the forming of thermoplastic and/or elastomeric resins with the use of a plastics processing machine provided with a screw.
In, for instance, a screw extruder the mass to be extruded is passed from a feed hopper via a conveying, melting and homogenization zone to the outlet opening. The shape of the outlet opening determines the profile into which the mass is formed. The melting of the mass takes place mainly in the melting zone. In the melting zone the mass is raised to a temperature above its melting point owing to the supply of heat through the wall and owing to the generation of heat in the mass in consequence of the work performed by the screw on the mass. In the melting zone the geometry of the screw is often such as to promote the generation of heat in the mass. After having left the melting zone, the mass is in a virtually completely molten condition, but internally it still shows great temperature differences. There are also great differences in the degree of distribution and possible reduction in size of additives through the polymer matrix. These differences in temperature, mixture and dispersion must be eliminated in the homogenization zone by means of a mixing and/or kneading effect of the screw on the mass, because the result of an inhomogeneous melt is that products of inferior quality are obtained.
The temperature curve of the mass in the extruder is such that, from the feed hopper onwards, there will be a rise, with a peak temperature in the melting zone or just after the melting zone if a dispersion section has been provided for, upon which there will usually be a fall towards the outlet opening through which the mass, still with a high temperature, leaves the extruder. The temperature at the beginning of the outlet opening is referred to as the final melt temperature. After the mass has left the machine, its temperature must be lowered by cooling to the ambient temperature or to a temperature suitable for a following process operation. Such a process operation may involve the blowing of film or the moleculair orientation of the polymer molecules as occurs in the monoaxial or biaxial stretching of the flat or tubular film and in the manufacture of bottles.
The high temperatures occurring in the extruder are noxious for the quality of the thermally sensitive plastics used. The necessary cooling of the extruded mass is not only expensive from an energy point of view, but in cases such as film blowing it may be highly limiting on the rate of production.
The invention provides a method for the forming of thermoplastic and/of elastomeric resins with the use of a plastics processing machine provided with a screw, which method eliminates the disadvantages mentioned to a major extent.
A device for the melting of plastics has become known from U.S. Pat. No. 4,218,146 with which, according to that application, a higher output of melt per unit of time can be obtained. This device, which can be used in both an extruder and the extruder section of an injection moulding machine, consists of a substantially cylinder-shaped body on which longitudinal grooves and ridges alternate, showing such a geometry that they form supply channels for the material to be melted and discharge channels for the melt. The grooves and ridges need not be truly longitudinal, so at an angle of 90.degree., but may be at a helix angle of 45.degree.-90.degree., preferably between 55.degree. and 75.degree.. The length of the cylinder-shaped body is, for instance, 1-20 D, preferably 3-15 D, specifically 8-12 D, where D stands for the diameter of the screw.
According to the invention, it has now surprisingly been found that, using the device described above, hereinafter referred to as melt torpedo, at the head of a plastics processing machine, incorporating the torpedo in its melting zone, final melt temperatures can be set which are tens of degrees lower than those applied in comparable circumstances in known plastics processing machines to reach certain quality specifications. Using the lower final melt temperature, properties could be achieved ranging from equivalent to improved at higher yields than were possible on the known machines under otherwise equal circumstances.
The invention is characterized in that the screw is provided, in the melting zone of the plastics processing machine, with a melt torpedo as formulated above, and in that a final melt temperature is chosen which is lower than that applied under otherwise equal cicumstances to reach the same quality specifications of the end product.